1. Field of the Invention
The invention relates to a solid state image pickup device, a camera, and a driving method of the solid state image pickup device and, more particularly, is suitable for use in a CMOS area sensor.
2. Related Background Art
In recent years, a CMOS area sensor in which a photodiode as a photoelectric conversion element and an MOS transistor as a switching element are formed as one chip is used as a solid state image pickup device. As compared with a CCD, the CMOS area sensor has such advantages that an electric power consumption is smaller, a driving power is smaller, a higher speed operation can be performed, and the like. Therefore, it is presumed that a demand for the CMOS area sensors will be increased in future.
A technique for widening a dynamic range of the solid state image pickup device by using such a CMOS area sensor has been proposed (refer to Japanese Patent Application Laid-Open No. 2001-186414 (corresponding U.S. Pat. No. 6,307,195)).
The proposed CMOS area sensor is constructed by forming a plurality of pixels, in a matrix shape, each having: a photodiode; a floating diffusion (hereinafter, abbreviated to “FD” as necessary) region; a transfer transistor for transferring carriers from the photodiode to the FD region; and a resetting transistor for resetting the FD region to a predetermined electric potential.
In the CMOS area sensor, first, a signal based on the carriers accumulated in the photodiode is read out and, thereafter, a signal based on the carriers which have overflowed from the photodiode and have been accumulated in the FD region is read out. The read-out signals are outputted through an analog amplifier.
In Japanese Patent Application Laid-Open No. 2004-335802, there is disclosed an MOS type solid state image pickup device in which a plurality of photosensitive units are arranged in an array shape onto the surface of a semiconductor substrate and a signal of each of the photosensitive units is read out every photosensitive unit, wherein each of the photosensitive units comprises: a first signal carrier detecting unit for detecting a signal corresponding to an incident light amount; and a second signal carrier detecting unit for trapping a part of excessive carriers in the first signal carrier detecting unit when a detection signal by the first signal carrier detecting unit is saturated and detecting a signal corresponding to an amount of the trapped carriers.
According to Japanese Patent Laid-Open No. 2004-335802, as shown in FIG. 2 of this Official Gazette, it is characterized by comprising a second signal carrier detecting unit (38) for detecting a part of electrons generated in a first signal carrier detecting unit (31) when those electrons are saturated. It has a structure in which a part of excessive carriers are trapped and the residual excessive carriers are abandoned into a vertical overflow drain. It independently has the first and second signal carrier detecting units.
However, according to the foregoing conventional techniques, an amount of carriers which are accumulated in the FD region cannot be controlled. There is, consequently, a problem that it is difficult to properly widen the dynamic range of the CMOS area sensor.
The invention is made in consideration of such a problem and it is an object of the invention to provide a solid state image pickup device which can properly widen the dynamic range, its driving method, and a camera using the solid state image pickup device.